The present invention relates to an electrical connection between a current output terminal and the electrodes of one polarity of an electrode plate group in an electrical storage cell.
An electrochemical cell or storage cell (these two terms being equivalent, the term storage cell will be used hereinafter) is a device for producing electricity in which chemical energy is converted into electrical energy. The chemical energy is constituted by electrochemically active compounds deposited on at least one face of electrodes arranged in the storage cell. The electrical energy is produced by electrochemical reactions during discharge of the storage cell. The electrodes, arranged in a container, are electrically connected to current output terminals to provide electrical continuity between the electrodes and an electrical consumer with which the storage cell is associated. The positive and negative current output terminals can be secured either onto the walls of opposing sides of the container of the storage cell, or onto the wall of a same side of the container.
FIG. 1 shows a sealed cylindrical storage cell of a known type.
Storage cell 1 comprises an electrode plate group 9 comprising alternating positive and negative electrodes flanking electrolyte-impregnated separators. Typically, each electrode is made up by a metal current collector also called a foil, carrying on at least one face thereof, the electrochemically active matter. The electrode plate group 9 is arranged in a sealed container 2 having a cylindrical wall closed off by a base 3 at one end thereof and covered at the other end by a lid 5 which carries the current output terminals 6 and 7. A first current output terminal, in the example the positive terminals 6, is generally welded onto the lid. A second current output terminal, in the example the negative terminal 7, passes through the lid; it is generally secured onto the latter by crimping, with seals 8 electrically insulating the negative current output terminal 7 from the lid 5.
The current output terminals 6, 7 provide electrical continuity between the electrodes and the external application with which the storage cell is associated. There exist several ways of electrically connecting the electrodes of a given polarity to one of the current output terminals on the container. One possible solution consists in employing a flat connection applied onto the collectors of the electrodes having the same polarity. This known solution is illustrated on FIG. 1; a flat connection 11 connects the positive electrodes of electrode plate group 9 together, and a conducting shaped part 12 connects this flat connection 11 to the bottom wall 3 of the container, the walls of the container being electrically conducting with the lid 5 and the positive terminal 6 welded onto the lid. Similarly, a flat connection 13 connects the negative electrodes of electrode plate group 9 together, and an elongated tab 14 connects this flat connection 13 to the negative current output terminal 7. Elongated tab 14 can form at least one bend in order to impart an elastic effect to the electrical connection between the negative electrodes and the negative current output terminal, and which compensates for variation in height of electrode plate groups from one storage cell to another.
Typically, the electrical connection to the terminal passing through a wall of the container—in the example of FIG. 1 this is the negative terminal—is assembled in the following way. Flat connection 13 is welded onto the collectors of the electrodes of a given polarity after which the elongated tab 14 is welded onto the flat connection 13 and onto the lower portion of the terminal 7 passing through a wall of the container. The elongated tab 14 is then bent inside the storage cell while the container 2 is being closed by the lid 5. An internal connection of this type is notably disclosed in European patent applications EP-A-1,102,337 or EP-A-1,596,449.
For high power applications, it is necessary to provide for heavy currents through the connections of the storage cell, for example currents greater than 100 amps. The choice will then fall on materials of high conductivity such as copper or aluminum to provide the connections. Typically, the negative electrode straps are of copper and the positive electrode straps are of aluminum for reasons of compatibility with the active matter, notably for lithium-ion type storage batteries. Thus, in lithium ion technology, the positive terminal is generally connected to the container of aluminum and the negative terminal is generally a terminal passing through a wall of the container and of copper. Copper is also generally chosen for flat connection 13 and elongated tab 14 to ensure good conduction of heavy currents and compatibility with the negative terminal of copper. It should be noted that flat connection 13 and elongated tab 14 may be one and the same part with a suitable shape which is directly welded onto the collectors of the electrodes that are folded as disclosed in European patent application EP-A-1,596,449.
The internal connection part (single part or elongated tab 14 welded to a flat connection 13) needs to be welded to the collectors of the negative electrodes of copper and then to the foot of the terminal 7 passing through a wall of the container. With lithium ion technology, the welds are typically performed by laser. Now, laser welding is not effective on copper as the laser beam is naturally reflected by copper. To get around this phenomenon, it is known to employ a nickel plate that is inserted between the copper and the laser beam in order to as it were “cheat” the laser and get it to transmit the laser heat energy to the copper to be welded. Welding the internal connection part consequently necessitates three parts: the actual connection part, a nickel welding plate to be placed against the portion to be welded to the collectors and a nickel welding plate to be placed on the portion to be welded at the foot of the terminal passing through a wall of the container.
It is also known, notably from European patent application EP-A-1,596,4496, to employ an internal connection part of nickel-plated copper. The nickel coating, several microns thick, is applied to prevent any oxidation of the copper part, and is not thick enough to sink the laser energy during welding. Supplementary parts for welding in nickel should always be employed with a connection part in nickel-plated copper.
FIG. 2 shows such an internal connection according to the prior art.
FIG. 2 shows diagrammatically the upper portion of an electrode plate group 9 to which an internal connection part 20 has been welded. It will be noticed in FIG. 2 that connection part 20 is covered by a nickel plate 21 over the portion which was welded to electrode plate group 9. The free end of this internal connection 20 is designed to electrically connect the foot of the terminal passing through the lid of the container when the electrode plate group will be introduced into the container (not illustrated). It will be also noticed in FIG. 2 that connection part 20 is covered by another nickel plate 22 over the portion which is to be welded to the foot of the terminal passing through a wall of the container. FIG. 2 also shows an exploded view of the three parts: internal connection part 20 and the two welding plates 21, 22.
Managing these three parts simultaneously complicates the manufacturing procedure for the storage cell, and represents a cost. Further, the nickel plates of around 0.5 mm thickness then remain inside the cell, although their only purpose is the welding of the internal connection. Now, the goal is to provide cells which are even more compact and lightweight; one consequently seeks to eliminate any part which plays no role in the operation of the cell.
There is consequently a need for a simplified internal electrical connection part which allows welding without the use of supplementary parts while still preserving low resistance to ensure passage of heavy currents.